mgr/nfs/utils: add helper func to check cephfs path

mgr/nfs/utils: add helper func to check cephfs path

this helper instantiates CephfsClient, however this was
initially planned in ExportMgr class in export.py but
due to make check failure where main python thread
experienced a dead lock which after several efforts
pointed at instantiation of CephfsClient in ExportMgr
was problematic, it was decided in order to achieve
singleton behavior, func has been added inside this
helper func that restricts instantiation using functool's
lru_cache.

Signed-off-by: Dhairya Parmar <dparmar@redhat.com>